The present invention is directed to an image-recording device having a number of light sources, which each have a distance si to an object line, the index i counting off the light sources, for generating image spots of the light sources in a projection line of the object line on a printing form, which moves at least with a velocity component v normally to the direction defined by the projection line and tangentially to the surface of the printing form, and having a triggering device for switching each individual light source.
To record images on printing forms, which form a plane or curved surface, whether it be in a printing-form exposure unit or in a direct imaging print unit of a printing press, a plurality of light sources, typically lasers, in particular diode laser arrays, are often used. Depending on the image information to be recorded, each individual light source or each individual emitter is switched on, and/or the variables influencing the light intensity are adjusted or preset for each printing dot to be set, so that each individual emitter generates an image spot having a specific light intensity. Frequently, an imaging optics for projecting the light emitted by the light sources onto the printing form is also optionally provided, often with the propagation direction of the light being changed, i.e., with beam formation. To achieve a precise and simple image recording of the printing form using the multiplicity of known image-recording methods, typically when the printing form is accommodated on a so-called printing-form cylinder or printing-plate cylinder, it is desirable for the image spots of the light sources to lie on the printing form in one projection line, in parallel with the cylinder axis. If the image spots are not positioned on one projection line or, in particular, not in parallel with the cylinder axis, image errors can arise on the printing form.
One main reason why the image spots of the light sources do not lie on one projection line on the printing form is that the light sources may, in fact, lie with adequate precision in one object plane, but are not positioned in this plane on one object line. The object line lies in the object plane. In principle, the object line can be a curve in the object plane having curved and straight sections. Without limiting universality, a straight object line is considered in the following. In other words: the individual light sources or emitters each have a distance si to an object line, i counting off the individual emitters, at least two light sources having a different distance to the object line. Typically, within the scope of the desired or required precision, this distance is not zero or cannot be disregarded.
This situation often occurs when lasers, in particular diode laser arrays, are used as light sources, which include one or a plurality of semiconductor components on a carrier element. Often, s-shaped or u-shaped deformations of a semiconductor component occur when a soldering process is used to apply the semiconductor component containing the emitter to a plane carrier element, such as a copper heat-sink member. This error, which is conditional upon manufacturing, is also described as the smile effect of the laser diode bar.
From the technical literature, devices are known for reducing or compensating for this smile effect which occurs in laser diode bars. U.S. Pat. No. 5,900,981, for example, describes an optical system for illuminating a spatial light modulator. The laser diode bar has a plurality of light sources, which emit light having different divergence in two mutually orthogonal axes, a so-called slow and a so-called fast axis, and which illuminate an array of microlenses. The array of microlenses collimates the light in the direction of the slow axis. An optical element adjacent to the microlenses refracts the light in the direction of the fast axis. An optical element focuses the emitted light in the direction of the slow axis, at a spatial light modulator, such that the light from each point-like light source irradiates the entire active surface of the spatial light modulator with light. A further optical element is provided which focuses the light in the direction of the fast axis, so that the desired spot size is generated in the direction of the fast axis. The optical system discussed in U.S. Pat. No. 5,900,981 uses natural and/or artificially introduced aberrations to reduce the sensitivity to the smile effect which occurs in most laser diode bars. The drawback associated with the optical system described in this document is the substantial outlay required for adjusting the microlens array.
U.S. Pat. No. 6,044,096 describes a compact laser diode array having reduced asymmetry. It provides for reducing the smile effect of the laser diode bar by allocating a plurality of thermal load elements to the plurality of individual diode light sources. The thermal load elements are similar to the individual laser diode light sources, with the exception that their power output is blocked. Each thermal load element is placed in direct contact with its individual assigned laser diodes, enabling an efficient heat transfer to take place. The operating temperature of the individual laser light source can be changed or adjusted by applying a variable current to the load element. Consequently, the relative position of two laser light sources to one another can be set by applying different currents to their respective, assigned thermal load elements. One disadvantage of using temperature to influence the smile effect is, for example, that the wavelength of the radiation emitted by the laser light source is changed. In addition, fluctuations in the temperature control lead to shifts in the position of the laser diode in relation to the array axis.
To generate image spots, the light sources are switched on and, after a certain time, switched off again. Typically, the emitters are activated by a trigger or tripping signal at a certain tripping instant. It is already known that, since the printing form moves with a velocity component v normally to the principal propagation direction defined by the light emitted from the light source, the image spot of a light source, depending on the tripping instant, is located on the printing form at a position whose coordinates are a function of the tripping instant. This fact is utilized, for example, in the device described in U.S. Pat. No. 5,174,205 for controlling an image-recording device for a printing form, which moves during the image-recording operation in relation to the image-recording device. The image information is stored in a first memory, while correction data for driving the light sources are contained in a second memory. The correction data are utilized to vary the time intervals between image-recording discharges from the light sources, to compensate for deviations between the measured position of the printing form relative to the image-recording head, and the actual position. In addition, U.S. Pat. No. 5,163,368 describes how a correction can be made for each image spot by using its coordinates on the printing form. These coordinate values are stored in a table. Using the correction devices described in these documents, changes in the size of the printing form can be considered in their coordinate directions, and compensated.
In this context, the German Patent Publication No. DE 26 53 539 is also mentioned. It describes the time-delayed triggering of an image-recording device having a number of light sources, whose image spots impinge at an angle on a printing form, thus not in parallel with the coordinate lines of the coordinate system corresponding to the geometry of the printing form. The offset from a coordinate axis caused by the oblique position of the image-recording device, assuming that all light sources are switched on simultaneously, is compensated by a corresponding change in the recording data. However, variably delayed clock pulses, thus trigger or tripping signals, can be applied to the assigned raster computer to compensate for the offset, while the original recording data are retained.
These related-art documents describing delayed triggering start from the assumption that an imaging device having a number of light sources is used to generate image spots on the printing form. These image spots are already situated precisely enough in one projection line on the printing form. At best, therefore, by varying the triggering of individual light sources, one is able to change or influence the position of the image spot on the printing form which moves at least with a velocity component v normally to the principal propagation direction of the direction defined by the light emitted from the light source.